Sensors have been under intense development during the last two decades, and many devices have been developed and commercially produced. Micromachined sensors (hereinafter just "microsensors") are sold worldwide in very large volumes, and are used in a wide variety of application areas. One of the most important manufacturing steps for the mass production of microsensors is the packaging which effects reliability, cost, and performance of such microsensors. Indeed, for many sensors, packaging is the least developed manufacturing step. In particular for a multi-sensor project, which is comprised of a variety of sensors and electronic chips, packaging problems are magnified. Packaging of sensors that are exposed to the ambient environment, such as pressure, flow and gas sensors, demands special attention. The pads and electrical connections to the sensor chip must be protected from the ambient environment while the sensing elements must be exposed to the ambient environment. The wire bonds and pads must be protected (e.g. encapsulated) while allowing the sensing element to be exposed. Traditionally, the wire bonds and pads have been protected from the environment, by depositing a protective coating on them. For example in a paper by D. J. Monk et al. titled Media Compatible Packaging and Environmental Testing of Barrier Coating Encapsulated Silicon Pressure Sensors, Proceedings of the 1996 IEEE Solid State Sensors and Actuators Workshop, the use of materials such as fluorosilicone gels and parylene as barrier coatings for protection of sensors from corrosive environments are mentioned. These methods however only address the problem of protecting the wire bonds and pads and do not address the overall issue of packaging of microsensors. In addition, these methods are not very effective for many applications.
Prior art for packaging of pressure sensors typically includes isolating the sensing element from the ambient environment, by securing an stainless steel diaphragm around the sensing elements. These diaphragms are generally either welded to the sensor package or held in place using some form of epoxy. In a paper by J. Mallon et al. titled Low-Cost, High Volume Packaging Techniques for Silicon Sensors and Actuators, Proceedings of the 1988 IEEE Solid State Sensor and Actuator Workshop, a method of packaging sensors for harsh environments is disclosed, where a metal isolation diaphragm is used between the pressure medium and the sensor. An incompressible liquid, for example silicone oil can be used then to transfer the pressure to the sensing element. Using this packaging method, the ambient environment condition information, such as pressure, flow rate etc., are transferred to the sensor, while protecting the sensing elements from direct exposure to the environment. However, a certain degree of accuracy is lost during the transfer between the diaphragm and sensing element. Also in applications where epoxies are used for bonding the stainless steel diaphragm, the epoxies may not be resistant to the operating environment, which would cause the packaging to fail in corrosive environment. Thermal mismatch between the epoxies and the diaphragms would also be a source of problem in the above mentioned packaging method.
Microsensors often have a low output signal, which requires the use of electronics for amplifying the signals received from the sensors. These electronic circuits need to be placed as close to the sensor as possible to minimize the unwanted effects of stray signals. K. Markus et al. in Smart MEMS Flip Chip Integration of MEMS and Electronics Proceedings SPIE Smart Structures and Materials, 1995 discuss a method where the electronics and the Micro-Electro-Mechanical Systems (MEMS) are fabricated on separate substrates, and are then connected using flip chip bonding. However they suggest that for applications where the front surface of the MEMS chip needs to be exposed flipping the MEMS chip onto the substrate cannot be used. To avoid this problem, two different methods were proposed in that paper. They claim that "This can be avoided by 1) using wafer through holes and having the solder wettable pads on the back of the chip, or 2) designing the MEMS die to be the larger bottom chip, placing the flipped electronics die to the side of the area requiring exposure." The problem with the first method proposed above is that the size of the package is increased unnecessarily. The other method proposed above, which is the method of using wafer through holes for transferring the electric connections from the front side of the sensor chip to the back side of the sensor chip, has been originally proposed by S. Linder et al. in Fabrication Technology for Wafer Through Hole Interconnections and Three-Dimensional Stacks of Chips and Wafers, Proceedings, IEEE MEMS Workshop, Oiso, Japan, January 1994. This method however results in adding to the processing complexity. Any step which increases the complexity of the process, leads to reducing the yield and increasing the production costs. In addition, using the above mentioned method, the top part of the sensor package including parts of the electrical connections are exposed which would cause problems if the sensor is required to operate in a corrosive environment.
In another paper by S. Bouwstra presented as an invited talk at National Sensor Conference, Delft, The Netherlands, March 1996, packaging of microsensors is discussed. The methods proposed in this paper are either the conventional die bonding approach (active side facing the surroundings) or a flip chip approach (active side facing the substrate). However the methods described in this paper all require multiple vertical feedthroughs with high wiring density in through holes, which is again a highly complex process which adds to the complexity of the packaging scheme. Another proposed idea in the paper mentioned above, involves bonding the transducer chip to a circuit chip, while establishing electrical connections between the two. Different techniques such as eutectic bonds, solder bumps and isotropic and anisotropic adhesives are discussed. Also mounting the entire assembly onto a substrate is mentioned, which would again require use of vertical through holes and result in adding highly complex fabrication steps to the packaging process. Once again as mentioned above, each step that adds to the complexity of the process will result in reducing the yield and increasing the cost of the process.
A method for packaging of chemical sensors is discussed in a paper by M. E. Poplawski et al. titled A Simple Packaging Process for Chemical Sensors, Proceedings of the 1994 IEEE Solid State Sensors and Actuators Workshop. The chemical sensors are packaged by flip chip bonding the sensors to glass substrates which contain fluid channels. The sensors are fabricated on silicon substrates, using a sequence of CMOS compatible semiconductor process steps. Precision screen printing is used to deposit a polymer gasket on the sensor chip, to act as a channel sealant. The sensor chip is inverted over the substrate and mounted over the flow channel in a way that the sensing element is positioned over the fluid channel. The polymer that was screen printed on the sensor chip, around the sensing element, seals the channel by acting as a gasket at the silicon-substrate interface. A similar concept is discussed in a paper by F. Kreibel et al. titled Flip Chip with Conductive Adhesives in Multichip Modules, published in the Proceedings of the Symposium on Conductive Adhesives in Microelectronics, Berlin, 1994. In this paper, an ion selective field effect transistor (ISFET) chip, is bonded to a fluidic capillary, using flip chip bonding. The fluidic capillary is formed by attaching a silicon spacer chip to a second silicon substrate, both containing through-holes. This silicon piece is attached to a glass substrate, thus forming a fluidic capillary. The ISFET chip is bonded to the fluidic capillary piece using flip chip bonding. A polymer gasket is screen printed on the ISFET chip, to seal the channel. Both of the papers mentioned above have been referenced in a book titled, Flip Chip Technologies, John Lau, Editor; Mc Graw-Hill, 1996.
In the two papers mentioned above, screen printing of polymers is used as a means for sealing off the flow channel, and protecting the rest of the package. This method may work for selected applications, however many problems can arise from the interaction of the polymer with the operating environment. One of the main problems is that most polymers exhibit a finite permeability to moisture. In most cases, the channel seal eventually fails because of interface modifications due to moisture penetration. In addition, the polymer might not be resistant to corrosive environments where the sensor might be required to operate in. Another problem arises from the fact that the screen printing process for fabricating the polymer gasket requires additional masks and processing steps. As mentioned previously, adding steps to any manufacturing process results in higher costs and lower yields.